lib: Note old ifindex on shutdown

lib: Note old ifindex on shutdown

If we have an interface configured in a daemon on shutdown
store the old ifindex value for retrieval on when it is
possibly recreated.

This is especially important for nexthop groups as that we
had at one point in time the ability to restore the
configuration but it was lost when we started deleting
all deleted interfaces.  We need the nexthop group subsystem
to also mark that it has configured an interface.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>